The field of portable electronic devices such as personal computers and cellular phones has progressed significantly. The portable electronic device needs a chargeable power storage device having high energy density, which is small, lightweight, and reliable. As such a power storage device, for example, a lithium-ion secondary battery is known. In addition, development of electrically propelled vehicles on which secondary batteries are mounted has also been progressing rapidly from a rise of growing awareness to environmental problems and energy problems.
In a lithium-ion secondary battery, as a positive electrode active material, a phosphate compound having an olivine structure and containing lithium (Li) and iron (Fe), manganese (Mn), cobalt (Co), or nickel (Ni), such as lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), or lithium nickel phosphate (LiNiPO4), has been known (see Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2).
Lithium iron phosphate is expressed by a composition formula, LiFePo4, and FePO4 which is formed by completely extracting lithium from LiFePo4 is also stable; thus, high capacity can be safely achieved with lithium iron phosphate.